Max (Myc-associated factor X) was first identified as a protein belonging to the basic helix-loop-helix/leucine zipper (bHLH/LZ) family of transcription factors that can associate with c-Myc in vitro and in vivo to form heterodimeric complexes with specific DNA binding activity (9, 10). Max exists in two forms as a result of alternative splicing of an exon, termed a, which encodes a nine-amino-acid insertion in the basic region (9). Both Max products have been shown either to heterodimerize with c-Myc or to form unphosphorylated homodimers and to specifically bind to the consensus sequence CACGTG (EMS [E-box Myc site]) (1,2,7,9,14,20,29,35,36). On the other hand, Myc forms homodimers only poorly under conditions that are physiologically relevant in vivo (29). Mutational analysis of Max indicated that the helix 1, helix 2, and leucine zipper (LZ) domains are critical for EMS DNA binding of homodimers, whereas the LZ domain is critical for binding of c-Myc-Max heterodimers (36). Cotransfection of Myc and Max expression vectors indicate that Max facilitates E-box-mediated transactivation by Myc at low doses, presumably via stimulation of c-Myc binding (2, 3, 15, 23). Conversely, overexpression of Max is inhibitory, suggesting that Max homodimers, which cannot transactivate, can compete for binding. More recently, two other members of the bHLH/LZ family of transcription factors, Mad and Mxi, have been shown to dimerize with Max product, but not with c-Myc, and bind efficiently to the E-box consensus sequence (4,5,42).Several other alternatively spliced forms of Max mRNAs have been reported (9,22,33,40,41). Makela and coworkers have identified an alternatively spliced Max mRNA containing an in-frame stop codon which would yield a putative protein (termed ⌬Max) truncated at the COOH terminus (33). Following in vitro transcription and translation, ⌬Max was able to associate with c-Myc and bind DNA in a sequence-specific manner. However no protein product was detected in the cell lines found to express ⌬max mRNA (33). Unlike Max, ⌬Max enhanced Myc transformation activity in a Ras cotransformation assay. Subsequently, Vastrik et al. (41) identified additional alternatively spliced max mRNAs of 1.9, 2.4, 3.0, and 3.5 kb, in addition to the predominant wild-type (wt) 2.3-kb species. These species displayed tissue-and cell line-specific expression in Northern (RNA) blot analysis (41). The 3.5-kb mRNA encoded a protein with biological activity similar to that of ⌬Max, but again the authors were unable to detect the putative protein product of this alternatively spliced max mRNA. Furthermore, in Xenopus laevis, the Max homologs XMax1 and XMax2 (resulting from alternative use of exon a) and two alternatively spliced mRNAs, termed XMax3 and XMax4, with an additional 81-base insertion in the C-terminal region of the coding sequence, termed exon b, have been identified (22,40). While these max mRNAs were found to be ubiquitously expressed during early embryonic development, and both in vitro-translated XMax2 and XMax4 proteins we...